Comfry mix with living things in the water

I would have thought so - you probably have something like mosquito larvae (do they swim in jerky bendy flip motions and rise to the surface?)  When you water, the larvae should die of dehydration and die - those left in the bucket of the comfrey feed will develop & emerge into full blown biting mosquitoes   

Put some vegetable oil in the barrel (not loads just enough to make a floating barrier)
The larvae will not be able to breath and the mosquito will not lay eggs in the oil slick.
Its harmless to you plants .

it only takes a drop or two to seal the surface...